Linear regulators or low-dropout (LDO) regulators are widely used in a variety of systems to provide a regulated voltage to other circuits in the system. In general, such regulators are required to provide and maintain a constant voltage across a wide variety of loads and/or operating frequencies in electrical applications. In particular, it is desirable to provide a stable and accurately regulated output voltage from an unregulated and many times noisy input voltage, i.e. typically the supply voltage of the regulator. The ability of a regulator to be immune to the noise injected in the input voltage is usually called PSRR (Power Supply Rejection Ratio).
PSRR describes the effectiveness of a regulator to eliminate output ripple caused by input/supply variations. Mathematically, PSRR is the reverse gain of the output ripple over the input ripple at a particular frequency. In general, it can also be defined by the amount of noise from a power supply that the regulator can reject, in other words, by measuring the amount of noise present on the power supply to the regulator which is transmitted to the output of the regulator. In case of a low amount of noise transmission, high PSRR is obtained, whereas a high amount of noise transmission leads to low PSRR.
An ideal linear regulator should provide a very high PSRR value across a wide variety of loads and/or operating frequencies. In particular, high PSRR values are desirable over the frequency range that is critical to the linear regulator, typically 10 Hz to 10 MHz. However, as a signal injected from devices supplied by the linear regulator may cause PSRR degradation at high frequencies, it is difficult to achieve high PSRR values across a wide range of operating frequencies.